The invention relates to a method for dewatering of sludge.
A sludge is defined as a mixture of solids and liquids. Usually the liquid in a sludge is water and therefore, usually the desirable portion of the sludge is the liquid. Separation of the solids from the water in a sludge is a common chemical engineering unit operation that comes under the general heading of xe2x80x9cwater treatmentxe2x80x9d or xe2x80x9cclassic solids/liquid separation processesxe2x80x9d. More specifically, this separation of the solids from the water in a sludge is usually described as xe2x80x9cdewateringxe2x80x9d of the sludge.
To conduct solids/liquid separation during water treatment, suspended solids in the sludge are removed from water by a variety of processes, including sedimentation, straining, flotation, filtration, coagulation, flocculation, and emulsion breaking, among others. The amount of solids in sludges being dewatered ranges from as little as several parts per billion of solids to large amounts, up to and including, actual chunks of suspended solids or oils. Sludges being dewatered may contain from about 99.75 weight percent water to about 40 or 50 weight percent water. Obviously, the more water in the sludge, the more processing it takes in order to xe2x80x9cdewaterxe2x80x9d the sludge. After a sludge has been dewatered, the liquid portion is usually processed further according to its intended end use, while the solids portion is typically properly disposed of in compliance with all applicable local, state and federal laws.
While strictly mechanical or physical means have been used to effect solids/liquid separation, modem chemical engineering methods typically rely on mechanical or physical separation techniques that are enhanced by the use of synthetic and natural cationic polymeric materials to accelerate the rate at which sludges can be dewatered. These solid-liquid separation processes include the treatment of sludges with cationic coagulant polymers that function to settle suspended inorganic particulates. Once settled out, the suspended inorganic particulates are readily removed from the liquid using mechanical or physical separation means. This separation of suspended inorganic particulate material makes the water usable for industrial or municipal purposes.
Dewatering of sludge is possible because particles in nature usually have either a cationic or anionic charge. Accordingly, these particles can be agglomerated, such that they can be readily physically separated from a liquid, by the addition to the sludge of a water-soluble coagulant or flocculant polymer having a charge opposite to that of the particles. This is referred to as a xe2x80x9cpolyelectrolyte enhanced solids/liquid separation processxe2x80x9d, wherein a water-soluble or water-dispersible ionically charged polymer is added to a sludge to neutralize the charged particles or emulsion droplets to be separated.
In a typical xe2x80x9cpolyelectrolyte enhance solids/liquid separation processxe2x80x9d, the dosage of these water-soluble or water-dispersible ionically charged polymers is critical to the performance of the process. Too little ionically charged polymer, and the suspended particles will not be charge neutralized and will thus still repel each other. Too much polymer, and the polymer will be wasted, or worse, present a problem in and of itself.
Lime or salts of iron or aluminum have been used to coagulate or flocculate solids; which facilitates the process of separation of these solids from sludge. There are known water-soluble or water-dispersible ionically charged polymers also used for this purpose. There are also known polymers that are of particular value as flocculants for suspensions of organic matter of a proteinaceous or cellulosic nature such as are to be found in sewage treatment effluents. In addition, there are synthetic polyelectrolytes, particularly certain cationic copolymers of acrylamide, that have been found to be of use in the field of sludge dewatering.
U.S. Pat. No. 3,409,546 describes the use of cationic polymers for dewatering of sludge. In this patent, the use of N-(amino methyl)-polyacrylamide polymers, is described in conjunction with other cationic polymers for the treatment of sewage sludges.
U.S. Pat. No. 3,414,514 describes the use of a copolymer of acrylamide and a quaternized cationic methacrylate ester for sludge dewatering.
Another class of cationic polymers used to dewater sludges is described in U.S. Pat. No. 3,897,333. The use of polyethyleneimines and homopolymers of cationic acrylates and methacrylates and other cationic polymers such as polyvinyl pyridines is also known.
Another example of a cationic polymer useful for sludge treatment is described in U.S. Pat. No. 4,191,645. In this patent, cationic copolymers prepared from a nonionic monomer, such as acrylamide, and a cationic monomer, such as trimethylammonium ethylmethacrylate methyl sulfate quaternary (TMAEM.MSQ) or dimethylaminoethylacrylate methyl sulfate quaternary (DMAEA.MSQ) are disclosed as being useful for sludge dewatering.
Further examples of polymeric treatments for sludge dewatering include the 1,4-dichloro-2-butene dimethylamine ionene chloride polymer disclosed in U.S. Pat. No. 3,928,448 and the block copolymers disclosed in U.S. Pat. No. 5,234,604.
Notwithstanding the variety of commercially available polymers that have been found to be capable of flocculating or coagulating sludges, there are various circumstances which tend to limit the usefulness of these reagents. While for certain sludges, economical treatments with these known reagents are feasible, more often sludges require very high and not cost effective dosages of reagents for successful treatment. Moreover, variations often occur in the composition of sludge from any one source. For example, variations in the supply of material to the sludge making process and/or in the oxidizing conditions that may be involved in the production of the sludge lead to a variety of particle types that must be removed. Furthermore, it is not uncommon to encounter sludges that are, for some reason, not amenable to flocculation by any of the known polymeric flocculating agents.
It is therefore desirable to provide additional chemical treatment materials and methods for the dewatering of sludge.
A process for dewatering of sludge, wherein said sludge comprises solids and water, comprising the steps of.
a) adding a cationic dispersion polymer and a microparticle to said sludge, wherein said cationic dispersion polymer can be added before or after or simultaneous with the addition of said microparticle; and
b) physically separating said solids from said water.
Throughout this patent application, the following definitions will be used:
All percentages given for components of polymers are to be understood to mean xe2x80x9cmol percentxe2x80x9d.
xe2x80x9cAcAmxe2x80x9d for acrylamide;
xe2x80x9cDMAEA.BCQxe2x80x9d for dimethylaminoethyl acrylate.benzyl chloride quaternary salt (also known as ethanaminium, N-benzyl-N,N-dimethyl-2-((1-oxo-2-propenyl)oxy)chloride);
xe2x80x9cDMAEA.MCQxe2x80x9d for dimethylaminoethyl acrylate.methyl chloride quaternary salt (also known as ethanaminium, N,N,N-trimethyl-2-((1-oxo-2-propenyl)oxy)chloride); and
xe2x80x9cDADMACxe2x80x9d for diallyldimethylammonium chloride;
xe2x80x9cExcess sludgexe2x80x9d refers to secondary biological sludge which needs to be removed from the system.
xe2x80x9cIVxe2x80x9d stands for intrinsic viscosity, which is RSV when the limit of concentration is equal to zero.
xe2x80x9cNalcoxe2x80x9d refers to the Nalco Chemical Company, One Nalco Center, Naperville, Ill. 60563, (630) 305-1000.
xe2x80x9cRSVxe2x80x9d stands for Reduced Specific Viscosity, which is an indication of polymer chain length and average molecular weight, which are indicative of the extent of polymerization during production. The RSV is measured at a given polymer concentration and temperature and calculated as follows:   RSV  =            [                        (                      η            /                          η              0                                )                -        1            ]        c  
xcex7=viscosity of polymer solution
xcex7o=viscosity of solvent at the same temperature
c =concentration of polymer in solution.
In this patent application, the units of concentration xe2x80x9ccxe2x80x9d are (grams/100 ml or g/deciliter). Therefore, the units of RSV are dL/g. For measuring RSV, the solvent used was 0.125 Molar sodium nitrate solution. The polymer concentration in this solvent was 0.045 percent. The RSV was measured at 30xc2x0 C. The viscosities xcex7 and xcex70 were measured using a CannonUbbelohde semimicro dilution viscometer, size 75. The viscometer is mounted in a perfectly vertical position in a constant temperature bath adjusted to 30xc2x10.02xc2x0 C. The error inherent in the measurement of RSV is about 0.5 dL/grams. When two polymers have similar RSV""s that is an indication that they have similar molecular weights.
xe2x80x9cRejectsxe2x80x9d are the lost fiber that is not suitable for the paper making process.
xe2x80x9cSludgexe2x80x9d is defined as a mixture of water and solids.
The method of the instant claimed invention is a process for dewatering of sludge, wherein said sludge comprises solids and water, comprising the steps of:
a) adding a cationic dispersion polymer and a microparticle to said sludge, wherein said cationic dispersion polymer can be added before or after or simultaneous with the addition of said microparticle; and
b) physically separating said solids from said water.
This improved method of sludge dewatering is applicable to any sludge from a papermill, a power plant and an industrial plant, wherein said industrial plant is selected from the group consisting of refineries, manufacturing plants, chemical plants, food and beverage plants and other industrial facility that creates sludges. The preferred sludge is that from a papermill and the preferred papermill sludge is that sludge comprising: a mixture of primary and secondary (biological) sludge which also contains about 10% fibers, wherein said mixture of primary and secondary (biological) sludge consists essentially of:
1. Foam from deinking plant;
2. Sludge from machines;
3. Waste water from the cleaning process;
4. Rejects from the dissolved air flotation (hereinafter xe2x80x9cDAFxe2x80x9d) units, and from the machines;
5. Rejects from the DAF units before the activated sludge plant; and
6. Excess sludge.
A variety of water soluble cationic dispersion polymer flocculants may be used in the practice of the invention. Both condensation and vinyl addition polymers may be employed. A preferred group of cationic polymers are those cationic dispersion polymers comprising acrylamide. In a preferred embodiment of the invention, the cationic dispersion polymers contain from about 10 mol % to about 80 mol % acrylamide. In a more preferred embodiment, the cationic dispersion polymers contain from about 30 mol % to about 60 mol % acrylamide.
In these cationic dispersion polymers comprising acrylamide, at least one other monomer must be copolymerized with the acrylamide. Typical of the cationic monomers that can be polymerized with acrylamide are the monomers DADMAC and DMAEA.MCQ. When these cationic acrylamide polymers are used they should have an RSV of at least about 3 dL/g and preferably the RSV should be within the range of from about 5 dL/g to about 20 dL/g. Cationic dispersion polymers useful in the process of the instant claimed invention include any of the following:
All of these cationic dispersion polymers are available from Nalco.
Microparticles useful in the instant claimed invention are selected from the group consisting of:
All of these microparticles are available from Nalco.
In practicing the process of the instant claimed invention the dose is based on the solids loading in the sludge stream. The dosage of the microparticle and flocculant can typically range from about 5 ppm to about 500 ppm. However, in sludge streams having higher solids loading and/or excess dissolved solids, the dosages can be much higher.
It is possible to practice the instant claimed invention by first adding the microparticle and then adding the cationic dispersion polymer to the sludge. It is also possible to practice the instant claimed invention by first adding the cationic dispersion polymer and then adding the microparticle. It is preferred in practicing the instant claimed invention that the microparticle be added first to the sludge, followed by addition of the cationic dispersion polymer.
It is possible to conduct the method of the instant claimed invention by adding the microparticle and the cationic dispersion polymer simultaneously. If simultaneous addition is selected, it is preferred that the sludge is divided into at least two separate streams and the microparticle is added to one stream and the cationic dispersion polymer is added to the other stream. Later in time, the two streams are once again merged and mixed together. In this way the cationic dispersion polymer and the microparticle both have a chance to react with the ingredients in the sludge, before they have the opportunity to react with each other. The sludge may be divided into two separate streams by any technique known in the art, such as, but not limited to, moving the sludge through a pipeline with a baffle in the center of the pipe and having points of addition for the cationic dispersion polymer and the microparticle be on opposite sides of the baffle.
The claimed invention provides for increased sludge dewatering. In situations, for example with papermaking sludges, it is possible to recover useful materials from said dewatered sludge. In fact, it is possible, by using the process of the instant claimed invention to recover usable pulp from papermaking sludges.
Increased water separation and drainage from the sludge also increases cake dryness. Increases in cake dryness result in substantial cost savings related to energy, sludge transportation, incineration or landfill costs. Better dewatering also increases the clarity of filtrates from which the clarified sludge cake is removed. This leads to better opportunities to increase the system efficiency without sending excess solids back into the waste plant.